Vehicles with a 5L40-E transmission are prone to poor line rise or high reverse pressure problems causing poor shift quality and clutch failure due to leakage at the boost valve assembly. Sonnax offers two replacement boost assemblies: the 55209-01K large ratio (higher line rise and firmer shifts) and the OE ratio 55209-04K for factory performance. Boost valves are hard-coat anodized to eliminate wear on the inside diameter of the sleeves. Boost sleeves are made from alloy aluminum specifically designed to resist wear. Large ratio 55209-01K or OE ratio 55209-04K assemblies are available. Kit salvages costly 5L40-E pumps. Note OE Ratio Kit Contents Valve Sleeve O-Ring Specs Dia.: 0.474" Dia.: 0.441"